To increase the life time of a wind turbine generator (WTG), it is well known that vibration levels can be monitored, so as to control operation of the WTG to avoid operating the WTG at conditions where high vibration levels are seen on any of such as: electric generator, gear box, and blades. Such control of the WTG locks the rotor speed to a rotor speed value, navigating around critical rotor speed ranges.
In practice, a WTG can be supplied with a predetermined set of rotor speed ranges which the control system avoids at long periods of time and at high loads. However, manufacturing tolerances and other factors influence actual critical rotation speeds for an individual WTG, and furthermore ageing of some mechanical components can also influence the actual critical rotor speed ranges for the WTG. Therefore, taking into account such manufacturing tolerances and ageing factors, a predetermined set of rotor speed ranges to avoid tend to cover a large range of possible rotor speeds, if a reasonable safety margin is taken into account. This means that ensuring a safe operation of the WTG will lead to a poor power efficiency, since the control system will navigate around the most power effective rotor speed in a large part of the operating time.